The present invention relates to a surface treated steel sheet used for an electric battery container, particularly to a surface treated steel sheet used for a container of a first battery such as an alkaline battery or a manganese battery, a battery container with the surface treated steel sheet and a battery.
Recently, performance of handy-carrier home electric products has been remarkably improved. Regarding a battery used for such products, it is required high grade and high performance. In a conventional art, in the case that a steel sheet is used for a container of the above described battery, a nickel plating steel sheet has been press-formed or a cold rolled steel sheet press-formed has been plated with nickel which has a good adhesive characteristic with respect to be adhesive to a surface of the steel sheet.
In order to improve a battery performance, an adhesive characteristic between an inner surface of a container and positive electrode active material is strengthened. Simultaneously, in order to enlarge an area of the inner peripheral surface of the container, it has been recommended various methods for reducing internal resistance of the container after finishing a step of forming a hard plating layer made from nickel-phosphorous alloy or nickel-tin alloy and so on.
In such a case, although the nickel-tin alloy is superior in view of reducing the internal resistance, there is a drawback. That is, a discharging characteristic is deteriorated while the alloy layer is contacted with alkaline liquid for a long time.
If it is selected and used metal or alloy having a good adhesive characteristic with respect to a steel container and positive electrode active material and of which degree is comparable to the nickel-tin alloy and a good discharging characteristic which is not deteriorated while the metal or alloy is contacted with alkaline liquid for a long time, the above drawbacks would be resolved and a life-span of such a battery would be extended.
A present invention is developed by utilizing a feature that Bismuth metal has an excellent anti-alkaline characteristic so that compounds of a plating layer is not solved even if the bismuth metal is contacted with alkaline solution for a long time and another feature that the metal is apt to be diffused so that an alloy layer can be formed easily. According to these features, it can be accomplished to provide a battery of which the discharging characteristic is not deteriorated for a long time by employing metal including bismuth metal as an inner surface of a battery container.
In order to accomplish the above purpose, a method according to the present invention is characterized in that a surface treated steel sheet for a battery container in which a nickel-bismuth alloy layer is formed on at least one surface of the steel sheet and the alloy layer is formed by an electrolyte plating method and an inner surface of the battery container is made of metal combined by a steel sheet for a battery container and a steel sheet in which the nickel-bismuth alloy layer is diffused. If a steel sheet made of only bismuth metal is used, the layer is apt to be peeled. Once the layer begins being peeled, the inner nickel layer contacts with alkaline solution directly. Such a phenomenon is unlikely.
By using such a steel sheet including a diffused alloy layer, an adhesive characteristic between a steel sheet used for a container and the alloy layer becomes proper and the discharging characteristic is not deteriorated since a solved amount of the alloy contacted with alkaline solution is remarkably less even if the alloy contacts with the alkaline solution for a long time.
Hereinafter, each embodiment of the present invention will be described in order.
(1) Steel sheet
When a battery according to the present invention is manufactured, a mild steel sheet should be prepared. Regarding the mild steel sheet, it is preferable that a cold rolling low carbon aluminum killed steel, an extremely low carbon steel sheet of which carbon content is equal or less than 0.003% and a non-aging extremely low carbon steel sheet in which niobium, boron and titanium are added. A reason why these mild steel sheets are used is an easy ironing process in accordance with the following steps. It is preferably that a thickness of the used steel sheet is about from 0.10 mm to 0.40 mm. Since it becomes easier to form a can in accordance with the following DI method (drawing and ironing) or the following DTR method (drawing thin and redraw).
(2) Nickel-Bismuth alloy plating
In order to form a nickel-bismuth alloy plating layer, it may employ a method in which nickel plating is firstly treated as a strike plating and then bismuth plating is treated so as to form the alloy. Alternatively, another method in which the nickel plating is firstly treated as strike plating and then nickel-bismuth alloy plating is treated thereon may be employed. Further, it may be employed another method in which nickel-bismuth alloy plating is treated without treating strike plating.
Regarding a nickel plating method, it may be employed a conventional non-electrolyte plating method or an electrolytic plating method. Further, regarding the electrolytic plating methods, a conventional watt bath, a sulfamin acid bath, a boric fluoride bath and so on can be employed as a plating bath. A thickness of nickel plating is preferably from 1 xcexcm to 10 xcexcm. The nickel plating is treated so as to improve an adhesive characteristic with respect to the following bismuth plating. The thickness is preferably equal or less than 10 xcexcm. If the thickness is less than 1 xcexcm, it would be difficult to form a diffused layer operated in the following treatment steps.
Bismuth plating may be treated in any conventional plating bath such as a perchloric acid bath, a pyrophosphoric acid bath, citric acid bath, a metasulfonic acid bath, a chloride Bismuth bath since basic components of the bathes are suitable for a purpose of the invention. In general, if organic components are added to a bath, a strict bath control is required so that a cost is apt to become higher. However, in the case of forming an extreme thin plating layer according to the present invention, the above drawbacks do not become so serious.
In a chloride bath, components thereof are only Bismuth chloride of 80 to 120 g/L, sodium chloride of 15 to 20 g/L and hydrochloric acid of about 200 ml/L. Although the components are very simple, processing steps are very complicated. For example, it is necessary to actuate a polar cycle in which a cathode is used for 10.5 seconds and the an anode is used for 5 seconds alternatively. In the case of providing a nickel strike plating layer, the nickel layer is solved when the plating layer is immersed in the bath so that the plating layer has to be immersed into the bath only for an anode cycle.
In the case of the metasulfonic acid bath, basic bath components are metasulfonic acid and bismuth oxide solved therein in which Bi3+ is finally 20 g/L and a free acid density is 2N. Further, diffusing agent and gloss agent are selectively added. For example, polyethylene glycol nonyl-phenyl-ether may be used as the diffusing agent and amine-aldehyde system solution of 20% may be used as the gloss agent. A reason why the gloss agent is added is to avoid for powdering a surface of a plating layer. If the gloss agent is included, a cathode potential is apt to be shifted to a potential level at which hydrogen is produced so that crystal of the surface of the plating layer can become finer. In order to obtain the most suitable effect, a ratio of the gloss agent with respect to the diffusing agent is within a range of 1 to 1.5. If the gloss agent or the diffusing agent is used solely, the above effect can not be obtained. A temperature of the bath is preferably from 20 to 25xc2x0 C. and an current density is preferably from 2 to 5 A/dm2. In the case of this type bath, a thickness of the bismuth plating layer is preferably from 0.01 to 1.0 xcexcm in general. The thickness of the plating layer can be controlled by adjusting a plating time. In the case of forming a plating layer on a steel sheet, nickel plating is treated on the both surfaces of the steel sheet as strike plating. In accordance with a purpose of the respective products, it may be selected one method for treating bismuth plating on an inner surface of the steel sheet or another method for treating bismuth plating on the both surfaces of the steel sheet.
(3) Formation of A Diffused Layer
A formation of the diffused layer is thermally treated. In the case of plating a nickel layer and a bismuth layer at two steps, a purpose of the thermal treatment is to form a nickel-bismuth alloy layer. By providing a diffused layer between iron material of a steel sheet and the nickel-bismuth alloy layer, the plating layer is not apt to be peeled easily while the steel sheet is processed and an inner resistance of an inner surface of a battery container can be reduced.
A thermal treatment is preferably treated in deoxidized/reduced protect gas in view of preventing an oxide film from forming on a surface of the alloy layer. Regarding the deoxidizing gas, so called inactive gas such as nitrogen, argon and neon is utilized. On the other hand, regarding the reduced gas, hydrogen, ammonia gas are suitably utilized. Regarding the thermal treatment, there are a box-type annealing method and a continuous annealing method. Any methods may be selectable. It is preferable that a temperature of the thermal treatment is equal or more than 300xc2x0 C. and a treating time is from 30 seconds to 10 hours. However, a condition of the thermal treatment is depending on a kind of a steel sheet. For example, if an extreme low carbon steel sheet of which carbon content is equal or less than 0.003 wt % is used, the temperature must be a constant temperature and the treating time must be a short time since a re-crystallization temperature of the steel sheet is relatively high.
(4) A formation of a Battery container
A battery container is formed by the DI (drawing and ironing) method or the DTR (drawing thin and redraw) method. In the case of the DI method, a thin steel sheet of which a surface is the nickel-bismuth alloy including the above described components is formed. A diameter of the battery container is slightly larger than an outer diameter of the battery container and a shallow drawing cup is prepared. The thin steel sheet is processed through a plurality of multi-step drawing dies coaxially arranged so as to decrease a drawing diameter in order. The steel sheet is continuously processed without occurring draw-down by pressurizing with a punch having a round corner at a front end.
In the case of the DTR method, as similar as the DI method, a shallow drawing cup must be prepared so as to draw the cup again. In the next step, a new drawing cup of which a diameter is smaller than the former drawing cup and a height is higher than the former drawing cup is prepared. Then, a high grade drawing cup is processed in order. In the re-drawing process, the cup is held by a ring-shaped pressing member inserted into the cup and a re-drawing die located under the cup. A re-drawing punch which is coaxially arranged with respect to the die is reciprocated in the pressing member. Re-drawing punches having different diameters are successively used in order. If necessary, other methods for producing a battery container can be employed.
(5) Manufacture of Alkaline Manganese Battery
Positive electrode active material of an alkaline manganese battery is produced by mixing manganese dioxide, carbon powder and alkaline solution. The manganese dioxide is an origin of an oxidation component so that it is required high activity and high purity. It is preferably to use electrolyte manganese dioxide in which MnO2 of equal or more than 91% is included.
Regarding characteristics of the carbon powder, it is required high purity and chemical stability, good conductivity, processability of mixed material and a characteristic for holding solution. For example, acetylene black, denatured products of carious carbon black, graphitized carbon black and synthetic graphite powder have been utilized as carbon black satisfied with the above requirements. Recently, synthetic graphite is mainly used. In the case of producing the positive electrode active material, a ratio of electrolyte dioxide manganese with respect to graphite is 5:1 to 10:1. Then, potassium hydroxide solution is added to them so as to mix the materials in accordance with a proper method.
On the other hand, in order to have a proper conductivity between a steel sheet for a battery container and the positive electrode active material, mixture of organic solvents such as graphite, thermosetting resin, methyl ethyl ketone and so on is coated on an inner surface of the battery container by a spray method and so on and then dried. The mixture is pressurized in a mold so as to form a donut-shaped positive electrode active material pellet. The pellet is attached to an inside of the battery container and pressurized. A cathode board on which a cathode collecting bar is spot-welded is attached to a battery container by neck-in processing at a predetermined location of a lower portion of an opening end of the battery container.
A purpose of a separator used for a battery is to prevent particles of negative active material and its products and particles of cathodic active material from moving mutually. Thereby, inner short circuit and self-discharging of a battery container be prevented by avoiding for producing the cathode products in the separator. Fiber material or non-woven fiber having anti-alkaline characteristic is used as the separator. Regarding the material, synthetic resin such as vinylon, polyolefin, polyamide and so on, linter pulp, mercerization wooden pulp or reproduced cellulose of which xcex1-cellulose content is equal or more than 98% are used.
One of these kinds of fiber separator is inserted along an inner surface of the positive electrode active material pellet pressurized at the battery container. Anode gel made of potassium hydroxide that zinc grains and zinc oxide are saturated is inserted into a battery container. Regarding the zinc grains, it is preferably atomizing powder of which a diameter is about 200 xcexcm. Regarding the gel, starch, cellulose derivative, polyacrylate and so on may be used. In the final, an insulate gasket is attached to the cathode member, inserted into the battery container, and caulked so as to manufacture an alkaline manganese battery.